The invention relates to cyclonic separation apparatus, particularly but not exclusively to cyclonic separation apparatus for use in a vacuum cleaner.
Cyclonic separation apparatus consists generally of a frusto-conical cyclone body having a tangential inlet at its larger, usually upper, end and a cone opening at its smaller, usually lower, end. A fluid carrying particles entrained within it enters via the tangential inlet and follows a helical path around the cyclone body. The particles are separated out from the fluid during this motion and are carried or dropped through the cone opening into a collector from which they can be disposed of as appropriate. The cleaned fluid, usually air, travels towards the central axis of the cyclone body to form a vortex and exits the cyclonic separator via a vortex finder which is positioned at the larger (upper) end of the cyclone body and is aligned with the central axis thereof.
The vortex finder usually takes the form of a simple tube extending downwardly into the cyclone body so that the vortex of exiting fluid is reliably directed out of the cyclone. However, the vortex finder has a number of inherent disadvantages. One of these disadvantages is the fact that there is a significant pressure drop within the vortex finder due to the high angular velocity of the exiting fluid. In an attempt to overcome this problem, centerbodies have been introduced into known vortex finders in combination with tangential offtakes in order to straighten the flow passing through and out of the cyclone. Some attempts have been made to reduce the swirl of the flow using fixed vanes. A variety of these attempts are illustrated in the paper entitled xe2x80x9cThe use of tangential offtakes for energy savings in process industriesxe2x80x9d (T O""Doherty, M Biffin, N Syred: Journal of Process Mechanical Engineering 1992, Vol 206). Other arrangements incorporating centerbodies or vanes are illustrated in WO 97/46323, WO 91/06750 and U.S. Pat. No. 5,444,982. In all of these pieces of prior art, the centerbody is wholly contained within the vortex finder or, if it is not, it projects only to a very minor extent into the cyclone body. This is because the single aim of the centerbody or vane is to remove the swirl from the flow within the vortex finder, rather than to stabilize it.
Centerbodies have also been introduced to cyclonic separators for other reasons. One such reason, illustrated in U.S. Pat. No. 4,278,452, is to expand the outgoing fluid so that an outermost annulus of fluid containing any particles remaining entrained is recirculated through the separator. However, by necessity, the major part of the centerbody must remain outside the vortex finder and therefore is incapable of stabilizing the fluid flow inside the vortex finder. Another use of a centerbody is to support an electrode by means of which a Corona discharge is produced within the separation zone of the separator. This enhances the separation efficiency within the separation zone but, because the electrode must incorporate angular or pointed areas from which the Corona will discharge, there can be no stabilization of the exiting fluid.
In CH 388267, use is made of a centerbody projecting out of a vortex finder to prevent bubbles of gas escaping from the main outlet of apparatus for separating solid particles and gas bubbles from a liquid suspension. The centerbody has an essentially flat end. The gas bubbles, which migrate to the vortex core during operation, are caused to exit the apparatus via the cone opening, which forms an outlet for the cyclone.
Another problem associated with vortex finders is the fact that, during operation of the cyclonic separation apparatus, the vortex core precesses around the interior of the vortex finder causing a significant amount of noise. The provision of a centerbody wholly within the vortex finder has been recognized as contributing to the reduction of the noise associated with the exiting fluid to a certain extent but no attempt has been made to make use of a centerbody to reduce the noise still further.
In domestic appliances such as vacuum cleaners, noise is always undesirable and there is an ongoing desire to reduce the noise associated with the appliance as far as possible. It is therefore an object of the present invention to provide cyclonic separation apparatus, suitable for incorporation into a domestic appliance, in which the noise level is improved. It is a further object of the invention to provide cyclonic separation apparatus in which the pressure drop appearing across the vortex finder is as small as possible. It is a still further object of the invention to provide cyclonic separation apparatus suitable for use in a domestic vacuum cleaner.
The invention provides cyclonic separation apparatus containing a cyclone body having at least one fluid inlet and a fluid outlet having a vortex finder. The invention also provides a vacuum cleaner incorporating such cyclonic separation apparatus. Further and preferred features of the cyclonic separation apparatus include a centerbody having a circular cross-section and a hemispherical, conical or frusto-conical end which protrudes beyond the lowermost end of the vortex finder to a distance at which the furthermost end of the centerbody is at least twice the smallest diameter of the vortex finder from the end surface of the cyclone body reduces the noise associated with the exiting vortex to an appreciable degree. The reduction has been found to be significantly better than in the case when the vortex finder does not protrude out of the vortex finder to any significant extent. It is believed that precession of the vortex core when bounded by the walls of the vortex finder causes pressure perturbations within the airflow which are manifested as noise. Hence it is desirable to stabilize this rotation completely before the exiting air enters the vortex finder. The extension of the centerbody into the core""s low pressure area before it reaches the vortex finder causes the core to stabilize before it reaches the vortex finder. The noise level is thereby reduced. Experimentation with specific apparatus has shown that, for specific dimensions of cyclone, vortex finder and centerbody, there are optimum distances from the upper surface of the cyclone to which the centerbody must extend. It will be clear from the description and examples which follow that it is not necessary for the centerbody to extend all the way up the vortex finder to the upper surface of the cyclone.